Fouling refers to the accumulation of airborne or waterborne biological materials on surfaces. Marine surfaces are especially prone to fouling, due to the affinity of marine organisms for areas at or below the waterline. In marine environments, fouling a involves surfaces on ship hulls, buoys, drilling platforms, pipes, and the like. Fouling build up on these surfaces can lead to a number of problems, such as increased weight or drag in the water, which, in the case of ships, can result in increased fuel consumption and operating costs.
The most common approach to prevention of marine fouling is through use of toxic antifouling coatings. The most commonly used antifouling coatings contain metallic toxicants, such as organo-tin or copper, which prevent marine organisms from attaching to the surface through release of the toxicant into the surrounding water. Such coatings may also contain an organic toxicant. A common form of these coatings, known as ablative antifouling coatings, wear away as the ship's hull passes through the water. This ablative action constantly brings fresh toxicant to the surface, until the toxicant concentration falls below a critical level, at which point the coating becomes ineffective. In order to restore the coating, the ship must be dry-docked and go through a recoating process.
A major concern of the use of antifouling coatings is the impact the leaching metallic toxicant poses to the environment. The use of organotin-based coatings has been found to kill, or at least severely restrict, the growth of marine life. This is especially true in areas of high ship traffic, such as harbors, bays, and estuaries. The use of copper based antifouling coatings is also being scrutinized for environmental hazards. It has been estimated that a ship having 3250 square meter hull area releases approximately 0.91 kg of copper per day, which is sufficient to bring approximately 18.9 million liters of sea water to toxic copper concentrations. ("Fluorinated Ship-Hull Coatings for Non-Polluting Fouling Control"; http://inel.gov/new/funding/serdp/p2prj005. html; May 30, 1996). Restrictions as to release of toxins into the environment are in place in certain areas. In addition to these problems, hulls coated with copper based coatings may experience the need for more frequent recoating than or anotin-based coatings.
Organic toxicants are considered to be less of a problem in this regard, since they tend to decompose to non-hazardous materials over time in water. Health hazards to dock workers exposed to organotin compounds and disposal of large quantities of toxic waste generated from removal of coatings during dry docking provide additional constraints to the use of or canotin-based antifouling coatings.
An alternative to the toxicant release approach is providing a coating or surface to which fouling organisms have difficulty adhering. Ideally, the turbulence created by the motion of the ship through water or simple cleaning methods would remove fouling organisms.
Pioneering work conducted by J. Griffith, "Nontoxic Alternatives to Antifouling Paints," Journal of Coatings Technology, vol 59 (755), 1987, pp 113-119, demonstrated that low surface energy coatings derived from fluoropolymers can function as fouling release coatings. Although these coatings demonstrated the principle of fouling release, certain marine organisms such as barnacles adhered strongly to the surface, requiring a cleaning step to remove them.
A. Beca and G. Loeb ("Ease of Removal of Barnacles from Various Polymeric Materials," Biotechnical and Bioengineering, v. 26, p. 1245-1251, 1984) studied the attachment of barnacles to a variety of polymeric surfaces and concluded that barnacles attached to a low surface energy surface were easier to remove than those attached to surfaces with higher surface energy. Researchers have also demonstrated through testing that marine organisms, in particular barnacles, attach more strongly to hard plastics than they do to soft elastomers.
A low surface energy approach was also demonstrated by Lindner, ("Low Surface Free Energy Approach In The Control of Marine Biofouling," Biofouling, 1992, Hardwood Academic Polyurethane Publishers, Vol. 99, pp. 193-205) who calculated coating surface energies based on contact angles with water and other liquids, and correlated them with contact angles critical to prevention of fouling by marine organisms. The higher the contact angle with water, the lower the surface energy of the coating surface. These materials were exemplified with oriented monolayers of perfluorinated surfactants fixed by polymers on the surface and by comb-like polymers with perfluorinated side chains. The preparation of a durable, water-borne polymer was not exemplified by this disclosure.
These studies confirm the need for a low surface energy surface, but also indicate that other factors, such as low glass transition temperature (&lt;-20.degree. C.) and elastomeric nature of the coating also play an important role in governing adhesion of marine organisms to polymeric surfaces.
Many commercially available silicones also contain leachable additives or residuals, which slowly move to the surface to form a weak boundary layer, resulting in easier removal of fouling organisms. Often, this additive is a silicone fluid.
While silicone coatings meet the requirements of low surface energy, low glass transition temperature, and elastomeric nature, there are major drawbacks to their use. These include poor abrasion resistance, tensile strength, and tear strength. These drawbacks result in susceptibility to mechanical damage. Also, silicone coatings do not exhibit good resistance to marine grasses and algae. Other potential problems with commercially available silicone fouling release coatings may include high solvent content and high material cost. Application cost may be high due to the necessity of multiple coats of dissimilar layers in order to achieve acceptable adhesion. Many of the silicone products are multi-component, requiring on-site mixing and pot life concerns.
Teflon.TM. filled materials, such as epoxies and vinylesters, are available, but they have a high glass transition temperature, are non-elastomeric, and are not low enough in surface energy to prevent strong adhesion of marine fouling organisms.
Polyurethanes have achieved commercial acceptance in surface finishing systems because of their overall balance of properties such as abrasion resistance, flexibility, toughness, high gloss, as well as mar and organic solvent resistance. Early commercial systems were either solvent based one-component reactive high solids prepolymers reacted with a second component, organic solvent-based moisture curing compositions or fully reacted urethane lacquers generally dissolved in alcohols and/or aromatic solvents.
In an effort to eliminate organic solvents and their associated emission and handling problems, waterborne urethane coatings were developed. Aqueous poly(urethane/urea) dispersions are binary colloidal systems in which a discontinuous polyurethane phase is dispersed in a continuous aqueous phase. Aqueous poly(urethane/urea) dispersions have been known for a long time. They are becoming increasingly important in coating and adhesive applications due to environmental and safety regulations of organic solvent based systems. Aqueous poly(urethane/urea) dispersions can be formulated using little or no co-solvent to produce high performance coatings and adhesives at ambient temperatures. They not only replace organic solutions but find applications in new areas as well. For instance, they are not aggressive towards plastic surfaces and provide excellent adhesion to glass and polymeric fibers due to their ionomeric nature.
Other advantages include: low toxicity (no free isocyanate), environmental acceptability, low viscosity at high molecular weights, tolerance to pH changes, elevated temperature stability, freeze/thaw stability, mechanical stability, compatibility with other materials, one component application, low temperature curing, excellent film forming properties, and typical polyurethane coatings performance.
The selection of starting materials suitable for making poly(urethane/urea) dispersions is essentially the same as that for conventional polyurethanes. Aqueous polyurethane dispersions are prepared with the aid of an external emulsifier or by forming polyurethane ionomers by incorporating internal emulsifier segments either into the backbone (e.g., quaternary ammonium groups) or pendant (e.g., carboxylate or sulfonate groups). The ionic groups can be anionic, cationic or zwitterionic and a wide variety of neutralizing counter ions may be used. When the polyurethane ionomers are dispersed in water, particles form which contain a core of aggregated soft segments with the ionic sites located predominantly on the surface. A very stable dispersion results. Stability of the dispersion is the result of the electrostatic repulsion of like charges between double layers of different particles. Particle size in stable film forming dispersions is 30-800 nm. The viscosity of the dispersion is dependent on the polyurethane particle size and solids content, degree of phase separation and independent of the polyurethane molecular weight (MW).
During film formation of polyurethane dispersions, water evaporates and the polyurethane particles coalesce to form a continuous film. The addition of plasticizers or high boiling coalescent solvents such as N-methylpyrrolidinone improve the film forming properties in some systems. Film formation properties also improve with elevated temperatures. The physical properties of the film are controlled by the selection of starting materials. The two areas where aqueous polyurethane dispersions tend to be inferior to solvent borne two-component polyurethanes are in water resistance and organic solvent resistance. The ionomeric nature of the polyurethane dispersions which makes them water dispersible, makes the film hydrophilic to some degree. Increasing the crosslink density of the polyurethane increases organic solvent resistance but highly branched prepolymers have very high viscosities and produce polymers with high glass transition temperatures (Tg) which have poor film forming properties.
Methods to improve the water resistance and organic solvent resistance of aqueous polyurethane dispersions have been developed. Grafting of hydrophobic unsaturated monomers (e.g., acrylates) onto polyurethane dispersions containing unsaturated polyester polyols is one method. The carboxylic acids on anionic polyurethane dispersions can be crosslinked by the addition of external crosslinkers. Polyfunctional aziridines, methoxymethylolated melamines or urea resins, carbodiimides, and polyisocyanates or blocked isocyanates are used. These become two-part systems with an associated pot life. Also, many of the crosslinkers are toxic and/or require elevated temperatures for cure. By incorporating reactive species onto the polyurethane backbone or on the interior of the dispersed polymer particle, self-crosslinking aqueous polyurethane dispersions can be made. Aqueous uralkyd resins can be produced which cure by free radical reactions of unsaturated hydrocarbons with atmospheric oxygen in the presence of metallic driers. Other methods of internal crosslinking involve final molecular weight buildup of the prepolymer at time of final application. High prepolymer viscosities and the poor film formation associated with highly crosslinked particles are avoided by these methods. These methods require that the film be baked, however. Most of these methods involve a "blocked" isocyanate which, at elevated temperatures unblocks and reacts with the carboxylic acid groups or amines. These self-crosslinking dispersions contain both reactive species as a "one-part" system. Aqueous polyurethane oligomers are available with only one reactive moiety. The most common of these are amino, hydroxyl or blocked isocyanate functional urethanes. A wide variety of coreactants are available.
Aqueous silane terminated urethane/urea dispersions are waterborne urethane/urea oli comers which have been capped with an alkoxy-functional silane. Silane terminated urethane/urea dispersions are normally stabilized by incorporating internal emulsifier segments into the backbone, typically carboxylate groups, with a corresponding trialkylammonium counter ion. When silane terminated urethane/urea dispersions are applied to a substrate and dried, the silanol groups condense to form a hydrophobic crosslinked coating.
Silane terminated urethane dispersions are described in U.S. Pat. Nos. 3,632,557; 3,627,722; 3,814,716; 4,582,873; 3,941,733; 4,567,228; 4,628,076; 5,041,494; 5,354,808 and European Patent Application No. 0305833 B1.
Curable water-borne silane terminated urethanes are described in Frisch et al., U.S. Pat. No. 5,554,686. These materials exhibit superior properties of water and sunlight resistance over standard, air dry water-borne urethanes. Because these materials cure as they dry, they also exhibit shorter tack free and dust free times than standard, air dry water-borne urethanes. Urethanes formed from fluorinated polyether polyols are disclosed in a list of polyols but are not exemplified. Silicone containing polyols are not disclosed. Neither fluorine nor silicone containing chain extender components or polyisocyanate components are disclosed. Low surface energy coatings are not disclosed.
Curable water-borne silane terminated urethanes with a backbone of alkoxy terminated polydimethylsiloxane (PDMS) diols are described in PCT application WO 95/21206 (Sengupta et al.) and EP 0742803 which are assigned to 3M. These materials are useful as low adhesion backsides for tape applications. These materials are not useful for marine applications or other applications where contact with water will occur (such as outdoor applications) due to their water absorbing properties. These materials do not contain silicone or fluorine in a chain extender or polyisocyanate component.